Power transmission belt for automotive applications

ABSTRACT

A power transmission striated belt including an elastomeric matrix (21) and a lengthwise supporting structure consisting of polyamide 4.6 twisted strands (20). The supporting structure (21) is selected so that the stress-elongation diagram of the belt exhibits an average slope ranging from 12 to 20 daN/% of elongation per width centimeter. The twisted strands are wound with an almost null nominal tension, and the curing operation and the cooling operation after curing are carried out without any belt tensioning.

BACKGROUND OF THE INVENTION

The present invention relates to a striated belt and, more particularly,to a V-profiled belt intended for automotive applications.

The Applicant markets striated belts with V-shaped teeth, the so-calledadapted-modulus belts, said belts comprising polyamide 6.6 twistedstrands and being intended for electric household appliances, such aslinen washers and dryers.

Such belts have a stress-elongation diagram, which characterizes theirmodulus of elasticity and the average slope of which, between 1% and 10%of elongation, is substantially equal to 5.5 daN/% of elongation, perbelt width centimeter and per strand.

Said belts are mounted, with a fixed distance between axes, bytensioning them and, once positioned, by releasing them (the so-calledautomatic mounting process or "snap-on").

Anyway, belts of this type but usable for automotive applications arenot on the market at the present time.

As a matter of fact, the use of twisted strands in a striated beltintended for automotive applications, so as to transmit power between anengine and a receiving device, such as an alternator, means a number ofproblems, especially owing to the acyclic property of the engine outputcurve, said acyclic feature being sharper for four-cylinder engines thanfor six- or eight-cylinder engines and being quite greater in dieselengines than in gasoline engines.

Since the electrical power requirements of automotive vehicles tend toincrease at present (air conditioning, etc.), the amperage ofalternators has to be increased too, and therefore their inertia, whichincreases proportionately the stresses imposed upon the belts.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a beltfor a motor vehicle, which exhibits a long service life even under highloads.

It is another object of the present invention to provide a belt capableof operating with engines having a marked acyclic feature.

It is still another object of the present invention to provide a beltfor a motor vehicle, allowing the filtering out of the high-orderharmonic components of the speed and voltage signal output by thereceiving device, more particularly when the latter is an alternator.

It is another object of the present invention to provide a belt capableof being incorporated in a drive system between a driving shaft and adriven shaft, by means of an automatic mounting operation ("snap-on"),advantageously without requiring a fixed tightener.

To achieve at least some of the above objects, there is provided a powertransmission striated belt which includes an elastomeric matrix and alengthwise supporting structure consisting of polyamide 4.6 twistedstrands, characterized in that the supporting structure is selected sothat the stress-elongation diagram of the belt exhibits an average sloperanging from 12 to 20 daN/% of elongation per width centimeter and perstrand, and in that the twisted strands are wound with a very smallnominal tension, or almost without any tension, and in that the curingoperation and the cooling operation after curing are carried out withoutany belt tensioning. More particularly, said average slope may rangefrom 12 to 15 daN/% of elongation per width centimeter and per strand.

It should be noted that the U.S. Pat. No. 4,701,377 and the EuropeanPatent Application EP-381 281 relate to a striated belt, in which thehigh elastic modulus twisted strands are made of polyamide 4.6, but saiddocuments do not suggest using such twisted strands for a low-modulusbelt.

As a matter of fact, it should be observed that the average slope of theuseful domain of the elongation curve is in the order of 30 daN/% ofelongation per width centimeter and per strand for the motor vehiclebelts known at present. Besides, the ultimate elongation of such beltsis generally below 10%.

According to the present invention, said average slope, between 1% and10% of elongation, is preferably substantially equal to 17 daN/% ofelongation per width centimeter and per strand.

The belt is advantageously characterized in that its length, as measuredon a test bench according to the ISO Standard 9981, is lower by 1%-6and, for instance, by 1%-5% or by 1%-4%, than the nominal length of saiddrive system.

In a preferred embodiment, the length of said belt, as measured on atest bench, is lower by 2%-3% than the nominal length of said drivesystem, the preferred value being substantially equal to 2%.

Advantageously, the belt has a stable operating tension ranging from 14to 20 daN/width centimeter/strand and, more preferably, ranging from 14to 17 daN/width centimeter/strand.

The twisted strands of the belt have a diameter d preferably rangingfrom 0.7 to 1.3 mm.

The space between twisted strands ranges advantageously from 0(close-turn lay) to 4 d, and preferably from 0 to 2 d.

The invention also relates to a manufacturing method for theabove-mentioned belt, characterized by the following steps:

a) forming, for instance on a drum, a belt blank, such step includingthe helical winding of twisted strands at a nominal tension either verylow or almost null;

b) curing the blank without subjecting it to any mechanical tensioning;

c) allowing the cured blank to cool down without subjecting it to anymechanical tensioning;

d) cutting the cured blank into individual belts.

The invention also relates to a method of the so-called "automatic" typefor mounting the belt onto a pulley of a drive system. It ischaracterized by the following steps:

i) positioning a mounting tool close to the pulley, so that at least aradial thrust area of the mounting tool rests against the pulley girth,the mounting tool including a ramp area extending from the groove of thepulley to a downward area, for the guidance of the belt right from thegroove while keeping it away from the pulley till it reaches a planespaced from said pulley face;

ii) radially moving said mounting tool by a lever effect, whilefollowing the pulley girth so as to fit the belt into the groove of thepulley while pulling on it;

iii) once the belt is positioned in the groove, disengaging the mountingtool.

Last, the present invention relates to a drive system between a pulleyintegral with a driving shaft of a car engine and at least one pulleyintegral with a driven shaft of a receiving device, such systemcomprising a belt as defined above and without any fixed tighteners.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and attendant advantages of the present invention willmore easily be understood by reading the following description of thepreferred embodiment thereof, only by way of a non-limitative example,taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a striated belt according to thepresent invention,

FIG. 2 shows a drive system provided with a belt according to thepresent invention,

FIG. 3a is a stress-elongation diagram corresponding, respectively, to abelt according to the present invention (Curve I) and to a belt (CurveII) not suitable for the present invention, as obtained on a test benchschematically shown in FIG. 3b,

FIGS. 4a and 4b show the comparative drive testing of a car alternatorusing a belt according to the prior art and to the present invention,respectively,

FIGS. 5a and 5b show a preferred embodiment of a mounting tool, capableof performing the automatic fitting of a belt according to the presentinvention, and

FIG. 6 illustrates the method for mounting the belt.

DETAILED DESCRIPTION OF THE INVENTION

An endless belt 10 comprises a lengthwise supporting structure 20 oftwisted strands embedded in an elastomeric matrix 21, such as a mixtureof polychloroprene rubber or natural rubber or any type commonly knownin the manufacture of power drive endless belts, for its good adhesionproperties. Such belt is advantageously of the "striated" type, i. e.its inner surface 22 is shaped like teeth 23, the pitch P of which isstandardized, as well as their triangular cross-section as shown (Vbelt) or their trapezoidal cross-section, each reference character H, J,K, L and M in the ISO Standard 9981 also defining the belt thickness, asmeasured between the tooth bottom 24 and the outer surface of the belt.The belt 10 cooperates with pulleys 11, 12, 13, 14, 15, 16, etc . . . ,the rims of which have a profile conjugate with that of teeth 23, thatis they have grooves arranged in the same way and with a cross-sectionwhich is similar to that, e. g. triangular or trapezoidal, of the teeth23, which they receive when the drive system is operating. The belt 10comprises two strands 17 and 18.

The V-shaped belts intended for automotive drive systems generally havesix teeth with a width l=3.56 mm, i. e. a total belt width L of 21.36mm.

According to the present invention, the twisted strands are made up ofadipamide polytetramethylene or polyamide 4.6. These are resins preparedby polycondensing 1.4-diaminobutane and adipic acid, for instance thosemarketed under the name of STANYL by the DSM Company. Such resins in theshape of yarns are then prepared into twisted strands.

Therefore, belts usable under severe operating conditions, such as thoseprevailing in drive systems for automotive applications, can be obtainedby selecting the twisted strand size and pitch p for the belt to exhibita stress-elongation diagram having, for instance between 1% and 10% ofelongation, an average slope in the range from 12 daN/% of elongationper width centimeter and per strand to 20 daN/% of elongation per widthcentimeter and per strand, and preferably substantially equal to 17daN/% of elongation per width centimeter and per strand.

Said slope may, more preferably, range from 12 and 15 daN/% ofelongation per width centimeter and per strand.

Besides, the method for manufacturing belts according to the inventiondiffers from that implemented in conventional belts for automotiveapplications, wherein the twisted strands are kept tensioned during thewhole manufacturing process.

In the conventional method, the twisted strands are first stretched andthen helically wound onto the drum carrying the gum sheets.

The tensioned twisted strands in the winding condition are still undertension when the blank is cured. For such curing, the blank is generallyarranged in a mold shaped like the belts to be produced, and then it ispressed by radial expansion onto the inner wall of the mold. Such radialexpansion increases the tension of the twisted strands.

Said tension disappears at the end of the curing process, when the blankis extracted from the mold and such tension cancellation results inshrinkage and thus in a smaller length of the belt.

In order to avoid such shrinkage, in said conventional method, thetwisted strands are tightened upon cooling from the curing temperaturedown to the ambient temperature, which means that they retain theirlength and that all shrinkage is prevented.

Such method has a number of consequences:

when the belt has been mounted, the operating temperature fluctuationsresult in length changes and, more especially when its temperatureincreases, the belt is brought back to its initial state, and thetwisted strands tend to shorten, which results in an increase in theservice stresses in the belt.

As the belt temperature is decreasing, the length tends to increase andthe belt slacks, which phenomena is even aggravated by the permanentstrain resulting from the twisted strand creep.

On the contrary, when a belt is manufactured according to the presentinvention, the twisted strands are helically wound on a drum carryinggum sheets at a nominal tension which should ideally be null but which,in practice, is very low or almost null, corresponding to a windingoperation under the lowest possible tension, being understood that somevery low or almost null (e. g. below 5 N) residual stresses alwaysremain, which fact cannot be avoided if only owing to the stressesresulting from the weight of the twisted strands.

Afterwards, during the pressure curing process, the blank is pressedonto the inner wall of the mold without any radial expansion andtherefore the twisted strands present an almost null residual stress.

After extraction and during the cooling process, the blank cools downwithout being tensioned. Once the cooling is over, the twisted strandspresent no tension at all.

Thus, an advantage of a belt according to the invention resides in thatthe operating temperature fluctuations have much less influence on thebelt that when said conventional method is implemented. Such smallerchanges in the belt length as a function of temperature also result in alower tension change, which prevents the belts from slipping on thepulleys, and reduces the generated noise.

One major problem in automotive applications is the acyclic featureowing to the nonuniform time distribution of the alternating pistonloads. This leads to a severe stressing of the belts and, moreparticularly, to a residual stress after stabilization (that is aftersome operating period) which is quite lower than in the case of drivesystems without an acyclic feature, such as the drive systems from anelectric motor.

Owing to such severe stresses, up to now it has not been possible toprovide belts having an elastic modulus suitable for the so-calledautomatic mounting of the belt in a car drive system.

Most remarkably, the belts according to the present invention, whichhave an average slope ranging from 12 to 20 daN/% of elongation perwidth centimeter and per strand, exhibit a stable operating tensionafter about ten hours of operation, which is in the order of 14 to 20daN per width centimeter and per strand.

Good results are obtained with twisted strands having a diameter rangingfrom 0.7 to 1.3 mm, the distance between twisted strands being in therange from 0 to 4 d, and preferably in the range from 0 to 2 d.

FIG. 3a shows the case of two driving belts in the automotive field,comprising six 3.56 mm-wide teeth and polyamide 4.6 twisted strands 20,manufactured according to the above-mentioned method (winding under analmost null nominal tension, curing and cooling without tensioning).

The belt according to Curve I (twisted strands, 470 dtex×6×3 ) has anaverage slope, between 1% and 10% of elongation (curve drawn using theleast-squares method), which amounts to 17 daN/% of elongation per widthcentimeter and per strand.

The belt corresponding to Curve II has an average slope, between 1% and10% of elongation, which amounts to 11 daN/% of elongation per widthcentimeter and per strand.

The elongation test, used for such diagrams, is carried out, forinstance, on a 200 mm-long strand retained between two jaws, the one 30being stationary and the other 31 being movable, using a dynamometer(refer to FIG. 3b).

The sole object of the above-mentioned average slope is to provide acharacteristic value for the stress-elongation diagrams, obtained bymeans of the least-squares method, but such curves should in no way beregarded as straight lines since, as a matter of fact, at each pointthey do have a slope, the value of which varies appreciably along thecurve.

A car belt according to Curve I and comprising six 3.56 mm-wide teethhas been mounted onto a drive system without any fixed tighteners. Themounting operation has been performed by stretching the belt to fit itonto the pulleys of the drive system. Once released, the belt is inposition with a 2% elongation, which corresponds to an initialelongation force of 80 daN per strand (refer to Curve I), that issubstantially 13 daN per tooth and per strand. After a few hours ofoperation, the stabilized tension has been measured. Its value amountedsubstantially to 6 daN per tooth and per strand. Such tension remainsstable and the belt exhibits a long service life.

A car belt according to Curve II and comprising six 3.56 mm-wide teethhas been mounted onto a drive system without any fixed tighteners. Themounting operation has been performed by stretching the belt to fit itonto the pulleys of the drive system.

After a few hours of operation, the stabilized tension has beenmeasured. Its value was quite below 6 daN per tooth and per strand, andthe belt was inclined to slip on the pulleys, which generated noise andthe service life of the belt was not very satisfactory.

Another restraint should be taken into account, especially as regardsthe filtering properties concerning the high-order harmonic componentsof the speed and voltage signal output by the receiving device. Suchphenomena is more particularly marked when high inertiae are present,i.e. when the motor vehicle is provided with an alternator.

FIGS. 4a and 4b illustrate this filtering phenomena. Each one shows, interms of time, the rotation speed of the main shaft (Curve M), here thatof a diesel engine at idle speed, and that of the shaft (Curve A) of areceiving device 14. The belt 10 transmits the speed, resulting from therotation of the pulley 11, to pulleys 12, 14, 15 and 16 of the receivingdevices, more particularly the pulley 14, which drives the alternatorhaving a torque of 3.5 10-3 kg.m².

Curve A in FIG. 4a has been obtained by means of a conventional belt,comprising polyester twisted strands, after a 48-hour operating time,the belt having reached a stabilized operating tension.

It can be seen that Curve A shows a repetitive anomaly 40 having a moreor less stable shape and corresponding to a slope interruption duringdeceleration. Such anomaly originates more particularly in the fact thatthe belt passes over to the pulley 14 the acyclic operation of theengine (cf. Curve M). This phenomena is all the more marked as theengine has a more acyclic property. By way of information, afour-cylinder gasoline engine has an acyclic property in the order of6-7%. Such property is in the order of 4-5% for a six-cylinder gasolineengine, but it ranges generally from 10 to 15% for a diesel engine. Somediesel engines even have an acyclic property as high as 20-25%. Saidanomalies 40 are due to the conjugate effect of the engine acyclicproperty and of the receiving device inertia, and they increase with theacyclic value of the engine and with the inertia of the receivingdevice.

The curve shown in FIG. 4b has been obtained using a belt according tothe invention, and corresponding to Curve I in FIG. 3a, after a 48-houroperating period. The belt has reached a stabilized operating tension of15 daN per width centimeter and per strand. It can be seen that saidanomaly 40 has completely disappeared. The belt according to theinvention has the quite remarkable property of being capable offiltering the high-order harmonic components of the speed signal outputby the receiving device.

The suppression of such anomalies 40 means a longer service life of thebelt.

The automatic mounting of a belt into a car drive system involves hightensile stresses. The mounting operation may be performed more easily bymeans of the tools to be described with reference to FIGS. 5a, 5b and 6.The mounting tool 40, shown in detail in FIGS. 5a and 5b, has a centralpart 41 provided with a square opening 42 for receiving a complementarypart 51 of an actuating wrench 50. It should be noted that the lever 50may be an integral part of the tool 40. The tool 40 has an inclinedconical part 43, at the top end of which a guiding flat surface 44 isprovided, and a L-shaped member having a leg 49 extending from thecentral part 42 and whose flat underside 48 is intended to rest on thesurface 61 of the pulley 60, and an end flap 46 bent substantially atright angle in relation to the leg 49, and the inner surface 47 of whichabuts against the rim 63 of the pulley 60.

The leg 49 is crowned by a guiding area 45 substantially in the shape ofan arc of a circle.

The object of the method according to the invention consists in fittingthe belt by a lever effect, using the mounting tool 40, which is guidedalong the girth (62, 63) of the pulley 60, whether the latter remainsfixed or is capable of rotating about a driving shaft received in itscentral opening 65.

The conical part 43, bounded by the inclined flat surface 44, and thepart 45 make it possible to guide, by a ramp effect, the side portion 27of belt 10 from the side portion 26 already accommodated in the groove62 of the pulley 60, up to the side portion 28, which stands apart fromthe surface 61. This is performed by bearing on the lever 50.

The procedure is as follows. The tool 40 is positioned so that thesurface 48 of the leg 49 rests on the surface 61 of the pulley 60, andthat the conical part 43 and the surface 47 of the flap 46 abut againstthe girth of the pulley 60, that is the rim 63. Of course, it would bepossible to obtain such thrust effect by pressing the tool 40 into thegroove 62 of the pulley 60.

The belt 10 is prepositioned in the ramp constituted by parts 43, 44 and45. By operating the lever 50 in the direction of arrow F, the tool 40is moved along the girth of the pulley 60 in the direction of arrow F',which results in the fitting of the belt 10 into the groove 62.

Should the pulley 60 be fixed, such travel of the tool 40 is performedwith a sliding motion. If the pulley 60 is capable of rotating, suchtravel of the tool 40 attends the rotational motion of the pulley 60.Anyhow, the lever effect combined with the ramp effect provide an easyfitting.

More particularly, such mounting operation takes place in the plane ofthe pulley 60 or close to it, which results in the fact that this methodis suitable for an exchange in the engine compartment, usually oflimited accessibility in automotive vehicles.

We claim:
 1. A power transmission striated belt suitable for automotiveapplications comprising:an elastomeric matrix; and a lengthwisesupporting structure consisting of polyamide 4.6 twisted strands;wherein the supporting structure is selected so that the belt has astress-elongation diagram which exhibits an average slope ranging from12 to 20 daN/% of elongation per width centimeter and per strand.
 2. Thebelt according to claim 1, wherein said average slope is equal to 17daN/% of elongation per width centimeter and per strand.
 3. The beltaccording to claim 1 wherein the belt has a length, as measured on atest bench according to the ISO Standard 9981, which is lower by 1%-6%than tne nominal length of a drive system.
 4. The belt according toclaim 3, wherein the length of said belt, as measured on a test bench,is lower by 2%-3% than the nominal length of said drive system.
 5. Thebelt according to claim 1, wherein the belt has a stable operatingtension ranging from 14 to 20 daN/width centimeter/strand.
 6. The beltaccording to claim 1, wherein the twisted strands have a diameterranging from 0.7 to 1.3 mm.
 7. The belt according to claim 1, wherein aspace between twisted strands ranges from 0 to 4 d.
 8. The beltaccording to claim 7, wherein the space between twisted strands rangesfrom 0 to 2 d.
 9. A drive system comprising a pulley integral with adriving shaft of a car engine and at least one pulley integral with adriven shaft of a receiving device, and wherein said pulleys carry abelt according to claim 1.